1. Field of the Invention
The present invention relates to a liquid ejection head for ejecting liquid. In particular, the invention relates to the improvement of the adhesion between an electrode layer for supplying electric power to a heat-generating portion for generating thermal energy for ejecting ink droplets and a resin layer provided on the electrode layer in an ink jet printing head for ejecting ink droplets.
2. Description of the Related Art
In recent years, with the advance of printing techniques, an ink jet printing apparatus also has been required to realize a printing with a higher speed and a higher image quality. In order to satisfy this requirement, it is required to increase the printing width of an ink jet printing head (hereinafter also called as printing head) and to arrange, in a printing head substrate as a component of the printing head, nozzles (also called ejection openings) with a higher density. To realize this, an increased number of heat generation resistance elements have been formed on a printing head substrate with a narrower pitch. This consequently requires an electrode layer for supplying electric power to the heat generation resistance elements to have a lower resistance in order to supply stable and uniform electric power to the respective heat generation resistance elements. Due to this reason, approaches by the formation of an electrode layer made of material having a lower resistance and by an increase of the thickness of an electrode layer on the substrate have been conventionally suggested.
One technique for increasing the thickness of the electrode layer as described above is a printing head substrate disclosed in Japanese Patent Laid-Open No. 2005-199701. This printing head substrate realizes a lower resistance by forming an electrode layer by plating with a gold (Au) having a thick thickness.
Generally, an electrode layer formed on a substrate for an ink jet printing head requires corrosion resistance. Thus, the electrode layer is made of noble metal such as gold (Au) and has thereon an insulating protection film as an upper layer so as to be protected from ink. This protection film may be formed, for example, by a method for forming an inorganic film made of SiN or Si for example by a vacuum film formation technique or the like. However, another method for forming an insulating protection film also has been currently considered by which an organic film made of polyimide or the like or resin such as polyether amide constituting an adhesion-improving layer for forming a nozzle is coated by the spin coating. The following section will describe the reason why organic material is used to form an insulating protection film.
In the ink jet printing head, in a process for forming nozzle components for forming an ink flow path and an ink ejection opening on a substrate, an organic film and a resin layer are layered on the substrate. Thus, if a film of organic material is formed on an electrode layer formed on a substrate, this film can function both as an insulating protection film and a nozzle component. This can consequently simplify the steps manufacturing a printing head and can reduce the material cost.
However, since an electric power wiring made of noble metal material is chemically-stable, this wiring has a poor adhesion to organic material. This causes a reduced adhesion at an interface between a nozzle component made of organic material and the electric power wiring. Furthermore, when an insulating protection film of an electrode layer is made of organic material, the insulating protection film consisting of organic material provided on the surface of the substrate is subjected to ink to expand or receives the stress caused by the heating of the heat generation resistance element and thus is easily peeled from the substrate. Thus, the ink jet printing head having an insulating protection film made of organic material has a disadvantage in that ink passes between the nozzle component and the electrode layer to enter the electrode and this ink causes the electrode to corrode or to be electrolyzed.